1. Field of the Invention
The present invention relates to a driving circuit. More particularly, the present invention relates to a light source driving circuit for driving a light-emitting element.
2. Description of Related Art
Light source driving circuits have been widely applied in various display devices, for example, backlight driving circuits of LCDs or LED driving circuits of handheld mobile devices. As display devices are increasingly used in various electronic products, the light source driving circuits have a great potential for development, and many advantageous features of the circuits, such as power conversion efficiency and stability, can be improved and enhanced continuously.
The architecture of a conventional light source driving circuit for driving an LED string 101 is as shown in FIG. 1. The LED string 101 includes a plurality of LEDs 102 strung together. The conventional light source driving circuit includes a DC/DC power converter 103, a capacitor 104, an output voltage feedback circuit 105, a timing control circuit 106 and a dimming circuit 107. The dimming circuit 107 is connected between a cathode of the LED string 101 and a common potential GND, and receives a pulse width signal PWM output from the timing control circuit 106. In addition, the output voltage feedback circuit 105 is implemented by two resistors 108 and 109 connected in series.
The DC/DC power converter 103 supplies a DC supply voltage to the LED string 101, and performs a feedback control according to a feedback signal FB received by the output voltage feedback voltage 105, so as to stably output a DC supply voltage. The timing control circuit 106 determines the width of the output pulse width signal according to the luminance set by the user. The dimming circuit 107 is turned on as the pulse width signal is at a high level, and is turned off when it is at a low level. By adjusting the width of the pulse width signal, the ON/OFF state of the dimming circuit 107 can be controlled, so as to further control the LED string 101 to whether or not permit a current to pass through.
Thus, when the luminance is set to high, the time that the pulse width signal remains at the high level is long such that the turn-turn-on time of the dimming circuit 107 is long, and the luminance of the LED string 101 is high. On the contrary, when the luminance is set to low, the time that the pulse width signal remains at the high level is short, such that the turn-on time of the dimming circuit 107 is short, and the luminance of the LED string 101 is low.
However, as the pulse width signal is at the low level, the dimming circuit 107 is turned off (i.e., the electrical path between the cathode of the LED string 101 and the common potential GND is turned off), and at this time, the voltage drop of the DC supply voltage supplied by the DC/DC power converter 103 completely falls on the dimming circuit 107. When the LED string 101 includes lots of LEDs 102 connected in series, the DC supply voltage supplied by the DC/DC power converter 103 will have a high voltage value.
Under this circumstance, if the withstand voltage of the dimming circuit 107 is not high enough, the dimming circuit 107 will be damaged by the DC supply voltage supplied by the DC/DC power converter 103. Moreover, the dimming circuit 107 with a higher voltage tolerancecan be used. However, the dimming circuit 107 with the higher voltage tolerance requires larger space and has a higher price, which not only increases the volume of the entire circuit, but also increases the cost of the circuit. Thus, the market competitiveness of this product is reduced.